Feldman-Sereda model

Fig. 8.4 Feldman-Sereda model of the structure of the C-S-H gel of Portland cement paste, showing C-S-H layers (lines), interlayer water molecules (crosses) and adsorbed water molecules (circles). After Ramachandran el al. (R32).

The Feldman-Sereda model was based on the studies of sorption properties, porosities and relations between water content and physical properties. Alone among the proposed models, it is clearly compatible with the microstructural evidence and with the probable relationships between C-S-H gel and crystalline compounds. It is incompatible with that of Brunauer, but not with the essential features of that of Powers and Brownyard in its original form if the nature of the gel porosity is reinterpreted. Calculations of bound water (Section 7.3.3) indicate that about a third of the gel porosity of the Powers-Brownyard model is interlayer space, the remainder being micro or fine meso porosity of the kind shown in Fig. 8.4. However, as that figure illustrates, the boundary between interlayer space and micropores is ill defined. 

Feldman (F33) explained his results in terms of the Feldman-Sereda model as follows. If the paste is D-dried, water is lost from the interlayer spaces. If the sample is immersed in water, this water is reabsorbed, and the total water porosity therefore includes the volurrie of the interlayer space. Methanol does not penetrate into this space, and helium does so only slowly, so that lower porosities are obtained. If the sample is equilibrated at 11% RFI, the interlayer spaces are largely filled, and the lower value is obtained irrespective of the fluid used. The helium porosities reported by Feldman were near to the calculated free water porosities. 

In the Feldman-Sereda model, the gel is considered as a poorly crystallized layered silieate and the role of water is much more complex (Fig. 7) than is recognized by the Powers-Brunauer model. Water does not re-enter the interlayer after d-drying. Water, in eontaet with the d-dried gel, acts in several ways ... 

Figure 1. Structure of C-S-H gel according to the Feldman-Sereda model.

There are two schools first of Bmnauer and the second one of Feldman and Sereda which, on the basis of different hypotheses, hy to explain the differences of cement paste porosity, measured with HjO sorption or other gases, principally N2 [35, 43, 48, 54]. These Itypotheses are connected rigorously with the proposed stmctural models for C-S-H phase. 

Fig. 5.34 Schemes of different C-S-H models a of Powers and Brunauer. b of Feldman and Sereda, modified by Daimon. c according to Wittmann...

In the model of Feldman and Sereda is assumed that the C-S-H gel structure is composed of single layers, randomly distributed. These layers, when approach each other, form the interlayer spaces similar to those occurring in the clay minerals, however, they are not ordered as in these minerals, but randomly placed. 

This mechanism is rejected entirely by Feldman and Sereda [64], as not consistent with their model. 

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